The invention is an improved electrode structure for use in electrochemical cells. The invention can be used in monopolar cells and in bipolar cells. The invention is useful in cells which employ permselective ion exchange membranes disposed between parallel, foraminous, metal anode and cathode electrodes. It is particularly useful in cells having substantially flat anode and cathode electrodes mounted at a distance from a fluid impermeable barrier layer which physically separates adjacent electrolysis cells. Such membrane cells are useful in the electrolysis of aqueous solutions of alkali metal chlorides; especially in the electrolysis of aqueous sodium chloride solutions. The cells may also be used in electrolyzing other solutions to make products such as potassium hydroxide, iodine, bromine, bromic acid, persulfuric acid, chloric acid, adiponitrile and other organic compounds made by electrolysis.
Except for the structures used for the terminal cells of a bipolar filter press cell series, the structures for intermediate cells in a series of cells are similar, repetitious, cell structural units which are positioned adjacent to each other and held together by a variety of different means. Examples of such cells operated in a series are disclosed in U.S. Pat. No. 4,111,779 (Sept. 5, 1978) and in U.S. Pat. No. 4,017,375 (Apr. 12, 1977). These patents are herein incorporated by reference for purposes of showing representative prior art and for showing how bipolar filter press cells are formed into and operated in a cell series.
The above features of a flat plate bipolar electrode type, filter press type electrolytic cell unit can also be observed in the following references U.S. Pat. Nos. 4,364,815; 4,111,779; 4,115,236; 4,017,375; 3,960,698; 3,859,197; 3,752,757; 4,194,670; 3,788,966; 3,884,781; 4,137,144 and 3,960,699. A review of these patents discloses the above described structural elements in various forms, shapes and connecting means.
Further description of monopolar electrodes used in a filter press series of electrolytic cells are given in; (A) U.S. Pat. No. 4,056,458 issued to G. R. Pohto et al on Nov. 1, 1977, and assigned to Diamond Shamrock Corporation; and (B) U.S. Pat. No. 4,315,810 issued to M. S. Kircher on Feb. 16, 1982, and assigned to Olin Corporation. Both of these patents teach the use of one type structure to support the monopolar filter press cell unit and they teach the use of other structures (a plurality of conductor rods or bars) to distribute electricity from and electrical source located outside the cells to the monopolar electrode members disposed within the cell. Other complexities of monopolar filter press series which call for many parts and many connections are observed from a study of these two patents.
To assure the effective use of substantially all of the surface of the electrodes in both monopolar and bipolar cells, it is desirable to provide electrical current to the electrode relatively evenly and without excessive resistance losses. To accomplish this, workers in the prior art have devised a variety of mechanical designs by which electrical current may be efficiently delivered to the electrode.
It is common practice to operate electrolytic cells with a membrane in contact with the anode or cathode (as in a finite gap membrane cell) or in contact with both anode and cathode (as in zero gap membrane cell). It is because of the close relationship with the electrodes that great care must be taken at the point where the welded connection is in close contact with the membrane. The complexity of electrical power distribution to the membrane makes it almost impossible to obtain a uniform current distribution.
It is expected that the weld points, which are the main electrical contacts, would have the highest concentration of electrical power. As the electrical power is transmitted across the planar surface of the electrode, the electrical power dissipates in intensity. This phenomenon is of course due to the resistance of the electrode material to the transmission of electrical power. Because of this, it can also be expected that the membranes in the area of the welds will be subject to higher concentrations of electrical power than the outward sections of the membrane away from the weld points.
As to the welded connections, electrical transmission through them is also dependent upon the percentage of the cross-sectional area of the supposed welds which is actually welded. Maldistribution of the amount of welded surface area from weld to weld across the face of the electrode is very difficult to avoid. Thus, with maldistribution of welds, there occurs again an additional maldistribution of electric power to the membrane.
Another undesired effect of this type of electrical contact is the blinding of electrolyte feed to the adjoining section of the active electrode. Since the area occupied by the weld can vary substantially, the membrane section affected can also vary. The greater the blinded area caused by the weld, the greater the area of the membrane surface that can experience the lack of electrolyte flow. This lack of electrolyte flow can cause a depletion of chloride ions, which causes the evolution of oxygen. Such a side-reaction, besides entailing a loss of current efficiency, has a detrimental effect on the active life of the anodes which rapidly loose their catalytic activity when oxygen is evolved. On the other hand, membranes are also particularly sensitive to the caustic concentration on the cathode side. For this reason, it is also highly desirable to maintain the caustic concentration across the contact areas on the cathode side of the membrane.
Still another key operational consideration is to minimize the stagnation of chlorine gas in the anolyte chamber. Since the attachment of the electrode can leave small voids at the stand-off means, and since these areas may be isolated from electrolyte flow by the area occupied by the weld, chlorine gas can become trapped in these voids. This trapped chlorine can then penetrate into the membrane and precipitate sodium chloride crystals. This build up of sodium chloride crystals within the structure of the membrane can cause small separations which can eventually lead to pin holes or delamination of the layers of the membrane, rendering the membrane less efficient or even inoperable.
The present invention allows the construction of the anode and cathode for both bipolar electrode type and monopolar electrode type cell series which greatly improves the current distribution across the lateral surface of the anode and cathode electrodes. The invention also allows the removal of excess heat of reaction at the contact points, the removal of stagnated chlorine gas, greatly reducing the risk of depleting the electrolyte at the contact points and neutralizing the effects of back migration of corrosive electrolytes, by creating an electrode structure which is simpler, much more flexible, and yet economical to manufacture and operate.